The pressure the pump can deliver is important only to knocking dirt and grime loose from the surface. PSI doesn’t tell us anything about its ability to carry the debris away from surface before it sticks again. For that, we need to look at the flow rate in gallons per minute (GPM).
More experienced power washer users will know about cleaning units (CUs), which is GPM x PSI. A 2,000 PSI machine with 2 GPM has 4,000 CUs. Combining PSI and GPM into one number gives a better idea of total cleaning power/ability.
Professionals know even the CU number is not enough to properly gauge what work a pressure washer can do compared to another unit…
…This is because there are 5 total pressure washer cleaning factors to know about:
- Pressure – Pounds per square inch (PSI) is the force of water hitting an area of a surface.
- Flow rate – Gallons per minute (GPM) is the water volume coming out the nozzle
- Heat – The higher the temperature of water, the more heat (energy) it has. Hot water pressure washers can increase the temperature incoming water by 130 degrees F (55C).
- Chemicals – Soap, detergent and other cleaning agents used to remove tough stains from specific surfaces.
- Nozzle spray pattern – There are many different nozzle spray patterns, each suited to specific tasks from getting gum off concrete to adding soap to a surface.
Take a look at this infographic to get the gist why each cleaning factor is important.
Pressure Washer Cleaning Factors – PSI, GPM, Heat & Chemicals – Infographic
Which is the most important single factor in pressure cleaning?
There is no scientific answer to this because there are too many factors and situations to look at.
All other cleaning factors the same (PSI, GPM, Soap)… Adding more heat is probably the biggest game changer in terms of cleaning efficiency. More heat in the form of hot vs. cold or warm water will scrub more debris and solids from a surface – and faster.
Just think of washing dishes in your sink.
The tap water is 50 PSI, 8 GPM, 60 degrees Fahrenheit and no soap added.
Now play with these factors a bit to see which gets the best result.
- Double the PSI what happens? 100 PSI is like blocking half the flow at the spout with your thumb. You are creating a nozzle that increases the PSI. Still, this won’t do much extra.
- Double the GPM what happens? This is going to waste a lot of water without doing much to clean on its own. A lot more mass of water is hitting the surface but the stains and stuck on cheese aren’t being knocked loose.
- Double the temperature what happens? If you go from 60 F to 120 F you are essentially going from a cool pool temp to a scalding hot jacuzzi. Thermal injuries will begin to occur after 5 seconds of contact with 140 F water. So 120 F is safe but very hot. Will it clean better than doubling PSI and GPM? Yes. I think it will – even at normal tap PSI and GPM.
- Add soap what happens? Soap may help if given time. It is probably better option than double GPM and double tap water PSI.
Jamey has been testing and reviewing top pressure washers for 7 years. He worked as a commercial pressure washer at a rendering plant for 3 years and all up has been using commercial and residential pressure washers for 15+ years. He is also a mechanical engineer and while working in the mining industry designed several turn-key light industrial vehicle wash pads.